Ion implantation is a standard technique for introducing conductivity-altering impurities into semiconductor workpieces. A desired impurity material is ionized in an ion source, the ions are mass analyzed to eliminate undesired ion species, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is implanted into a workpiece. The energetic ions in the ion beam penetrate into the bulk of the workpiece material and are embedded into the crystalline lattice of the workpiece material to form a region of desired conductivity. Ion beam energy is an important parameter that is controlled during this ion implantation. Failure to control ion beam energy may result in the ions being implanted to an improper or undesired depth in the workpiece.
Measurement devices, such as Faraday cups, have been used in the past to measure beam current in ion implanters. Faraday cups are typically metal or graphite devices that catch charged particles, such as ions or electrons, in a vacuum. As the charged particles enter the Faraday cup, the resulting current is measured to determine the number of charged particles impacting the Faraday cup. Ion beam current, or the number of charged particles over a particular period of time, in an ion implanter may then be calculated using equation 1:N/t=I/e  (1)where N is the number of ions observed, t is the length of time in seconds, I is the measured current in amperes, and e is the elementary charge in coulombs. Elementary charge is the electric charge carried by a single proton or the negative of the electric charge carried by a single electron. This constant is approximately 1.6E-19 C.
FIG. 1 is a cross-sectional view of a first embodiment of a measurement device. The measurement device 101, which is a Faraday cup in this instance, has a plate 103 and a collection cup 102. The plate 103 defines an aperture 106 and is grounded in this embodiment. A threshold (represented by the dotted line 104) exists between the plate 103 and the collection cup 102. The measurement device 101 will output a signal to the current measurement device 105 when a is charged particle in the ion beam 100 crosses the threshold between the plate 103 and the collection cup 102 rather than, as might be assumed, when the charged particle strikes the back of the collection cup 102. FIG. 2 is a figure illustrating current over time for the first embodiment of the measurement device. At point 110, the charged particle in the ion beam 100 approaches the aperture 106 in the measurement device 101. An initial image charge appears and drives a current to the current measurement device 105. At point 111, the charged particle is at the threshold. At point 112 the charged particle passed through the aperture 106 and the threshold, creating most of the image charge. At point 113 the charged particle approaches the back of the collection cup 102. At point 114 the charged particle strikes the collection cup 102 and is neutralized by the image charge. No signal appears in the current measurement device 105 after point 113.
The measurement device 101 will integrate the electrical current that is measured by the current measurement device 105 to calculate the current of the ion beam 100. While prior art Faraday cups, such as measurement device 101, measure current, such a Faraday cup cannot calculate the energy of the charged particles, such as in the ion beam 100. This is an important parameter within the semiconductor manufacturing market as it determines the depth of penetration of the charged particle. Accordingly, there is a need in the art for an improved measurement device that can measure the energy of charged particles in a charged particle beam.